Hematopoietic stem cells (HSCs) have long been regarded as restricted to formation of blood cells of both the lymphoid and myeloid lineages. HSCs residing in the bone marrow microenvironment can repopulate the hematopoietic system of irradiated transplant recipients for the lifetime of the animal making HSCs important targets for gene therapy applications. However, recent studies have suggested that a subfraction of HSCs may have the ability to contribute to diverse cell types such as neurons, hepatocytes, and myocytes. These surprising findings contradict the long-standing dogma and have opened an important new field of investigation into identification and characterization of these potential pluripotent stem cells. Recent studies have shown the therapeutic efficacy of bone marrow cell populations following induced myocardial infarction. While these studies have suggested that HSCs have potential to become functional cardiomyocytes following direct injection, bone marrow transplant, or cytokine-induced mobilization, these studies are highly invasive and require delicate mouse surgery. We propose to establish a simpler method by inducing cardiomyopathy following injection of mice with doxorubicin, thus providing an injury model that is also clinically relevant for cancer chemotherapy with anthracycline drugs. This approach will be used to test for hematopoietic stem cell cardiac differentiation potential and will determine whether activation of the gp130/STAT3 signaling pathway promotes HSC contribution to cardiomyocyte regeneration. The gp130/STAT3 pathway activates an embryonic gene expression program resulting in hypertrophy and increased levels of cardiotrophin and vascular endothelial growth factor in the heart. Aim #1 will determine whether transgenic mice overexpressing STAT3 in cardiac cells will provide an optimal microenvironment to stimulate maximal cardiac differentiation from bone marrow derived precursor cells. Aim #2 will determine whether overexpression of STAT3 in donor bone marrow cells will provide a cell intrinsic selective advantage for cardiac differentiation. Prospective identification of cells with this potential could circumvent concerns about using human embryonic stem cells. Generation of systems for efficient assay of transdifferentiation potential are needed since this developmental plasticity might be exploited to create new stem cell therapies. Characterization of signaling mechanisms involved in stem cell-mediated tissue regeneration represents a new area of research that will be critical for understanding the regulatory mechanisms behind the response of stem cells to diverse environmental stimuli.